The present invention generally relates to electrographic imaging machines, such as printers and copiers. More particularly, the present invention relates to a braking device for a recording medium to halt a sheet recording medium or substrate along its transport path.
Pneumatic braking devices for a recording substrate are used in electrographic printing or copying machines. The recording a mediums or recording substrates used in these machines typically are paper or plastic films in single-sheet or web form. On their path through the electrographic printing or copying machine, the recording substrates are transported between transport rollers over gliding surfaces. In some cases it is necessary to brake the recording substrate on the gliding surfaces and to smooth it in cooperation with the transport rollers. At such locations, suction openings are provided in the gliding surfaces and are coupled to a vacuum-generating device.
An example of such an application is the fixing device of an electrographic printing or copying machine. There, a recording substrate provided with toner images must run through a fixing station. In so doing, the toner image is melted into the recording substrate. Such a fixing station, known for example from U.S. Pat. No. 4,147,922 or JP-Abstract Vo. 13, No. 120, Mar. 24th, 1989 (JP-A-63-292177), can comprise two rollers--a fixing roller and a nip roller--of which at least one is heated and one is motor-driven, and a preheating device, for example a heatable saddle, arranged in front of the rollers. The satisfactory functioning of the fixing process requires that the recording substrate rests closely on the preheating saddle. For this purpose, the recording substrate must be tensioned over the saddle. In order to achieve this, a braking device can be provided in of the saddle, seen in the movement direction of the recording substrate.
The pneumatic braking device has, in such applications, the advantage that the recording substrate is sucked onto the gliding surface of the braking element of the braking device for braking and hence the braking device engages on only one side of the recording substrate. The other side of the recording substrate, for example the side of the recording substrate on which the toner images are arranged, is not impaired by the braking device.
From U.S. Pat. No. 4,172,301, a measure is known to match the suction width of the gliding surface to a parameter, specifically the width of the recording substrate. This reference is incorporated herein by reference in its entirety. In this arrangement, in accordance with the width of the recording substrate, specific suction openings in the gliding surface are closed, while the suction openings located in the region of the recording substrate remain opened. By means of this measure it is ensured that the fixedly set value of the vacuum, which is generated with the aid of the vacuum-generating device, can be maintained in the case of different recording substrate widths. Further measures involving the vacuum are not provided.
Patent Abstracts of Japan, Vol. 14, No. 179 (p-1034) & JP,A,02 028 677 disclose a fixing device having pneumatic braking element and preheating saddle of a printing or copying machine. The suction air in the suction openings of the preheating saddle can be switched off by means of valves. Switching off the suction air is carried out in an operating condition of the printing or copying machine in which no recording substrate transport is taking place. As a result, the recording substrate lifts off from the preheating saddle. Warping of the recording substrate in this operating condition is prevented. A possibility of varying the vacuum as a function of parameters of the recording substrate is not provided.
With reference to the gliding surface of the preheating saddle of a fixing device, special problems arise. In the known fixing devices, it has previously been assumed that it is necessary to preheat the recording substrate, comprising paper as a rule, very rapidly over a relatively short path with the aid of the preheating paddle and then to fix the toner image on the recording substrate via the rollers. However, it has been proved that a rapid heating up of the recording substrate over a short path leads to a high loading of the recording substrate. This loading is expressed in a deformation, a warping or an ageing of the recording substrate. Recording substrates comprising paper also exhibit a non-uniform loss of water during the passage through the fixing station. Hence, post-processing of the recording substrate by cutting or sorting is made more difficult or a non-uniform fixing of the toner images and thus an impairment of the quality of the print occurs.
Remedies can be supplied by slowing down the heating up of the recording substrate. However, a necessary lengthening of the preheating saddle is associated with a slowing down of the heating up in the case of a constant printing speed. In the case of this lengthening of the preheating saddle it is essential to ensure good thermal transfer between recording substrate and the saddle gliding surface. Good thermal transfer is only achieved if direct contact can be produced between recording substrate and saddle gliding surface. At high printing speeds and in the use of pre-folded recording substrates or those of non-uniform thickness, fluttering movements of the recording substrate can occur in the region of the saddle. As a consequence, the recording substrate lifts partially off from the saddle, and this impairs the thermal transfer.
Endless paper printers, in particular, must be able to process an extremely large spectrum of recording substrates in paper form. The recording substrate has, for example, a paper basis weight (grammage) of 50 g/m.sup.2 to 160 g/m.sup.2. Coated and uncoated papers, recycled papers, plastic-coated papers, and long-fibered and short-fibered papers are processed. In addition, every year grades of paper with new properties come onto the market, which must be processed by the printing or copying machines in an optimal manner without hardware change.
The material of the recording substrate contains various components which are released on heating. The components released in the form of vapor or gas are precipitated in the printing or copying machine and cause disturbances there. In the heating of paper, for example, steam is released which can cause corrosion.